Conventional production methods for optical disks include, for example, a method in which first an original metal plate (stamper) is created with a negative image of the pattern to be formed on a plastic substrate that forms an optical disk. Then, a substrate is produced by injection molding from the stamper. The stamper is produced, for example, by the method illustrated in FIGS. 11A-E. This particular method is described below.
Generally, an original glass plate is used to create a stamper, and a photoresist 112 is applied to the glass plate 111 as shown in FIG. 11A. Then, as shown in FIG. 11B, a pattern based on the desired information is exposed onto the photoresist 112 using a laser cutting machine. Next, the photoresist 112 is developed, and exposed areas are removed to form a relief-and-indentation pattern on the surface, as shown in FIG. 11C. Then, as shown in FIG. 11D, conductivity treatment is applied (a metal film is vacuum-formed) to the photoresist 112 where the pattern is formed, and nickel plating 113 is added. Next, a nickel stamper 114 is created by separating the nickel plating 113 from the original glass plate 111 on which the pattern was formed, as shown in FIG. 11E.
Injection molding is a method that uses the stamper to produce plastic substrates. Generally, a polycarbonate is used as the plastic substrate material.
Japanese laid-open patent publications No. 1-150529 and No. 4229430 respectively disclose an optical disk in which a cyclic olefin copolymer is used. These publications disclose that the use of a substrate for an optical disk made of a given ethylene/cyclic olefin polymer and a set item of given composition for its adhesive layer allows the production of an optical recording medium that has an adhesive layer with excellent in adhesive strength, water-proofness and moisture-resistance, as well as a substrate layer with superior dimensional stability even under high-temperature and high-humidity conditions, as well as under normal conditions. Further, this substrate has the advantages of excellent heat-resistance and transparency, and it has no color and only a small birefringence, and it hardly warps.
A production method for optical disks using UV-curing resin is disclosed in Japanese laid-open patent publication No. 53-86756. This publication discloses a method that uses UV-curing resin to transfer a pattern to a polymethyl methacrylate, a polycarbonate, etc. a master made of nickel by electroforming (hereinafter, a "master" is used as the equivalent of a stamper).
Alternatively, a method that uses a silicon wafer to create a master for optical disks is disclosed in Japanese laid-open patent publication No. 61-68746. According to the method as disclosed in this publication, silicon oxide is formed on a silicon wafer after which a photoresist is applied and exposed. The photoresist is developed and then the silicon oxide is etched to create a master.
Also, replicas for molding can be produced by other methods from a master made of silicon. Japanese laid-open patent publication No. 4-299937 discloses a method for creating a master by directly etching a silicon wafer. Also, Japanese laid-open patent publication No. 5-62254 discloses a method that uses UV-curing resin to transfer a pattern from a master made of a silicon wafer to a plastic substrate.
Methods disclosed in Japanese laid-open patent publications No. 4-310624 and No. 4-311833 have the advantage that a master produced using a silicon wafer yields optical disks that have enhanced recording density.
Still, it is necessary to highly densify a recording pattern formed on an optical disk in order to increase the amount of data recorded on the disk. To achieve high densification by conventional methods, it is necessary to increase the density of the pattern formed on the stamper. For this reason, it is common to employ a method that reduces the laser wavelength of a laser cutting machine.
However, gas lasers such as helium cadmium lasers with a wavelength at the level of the 400 nm or argon lasers are used in current laser cutting machines, and because the laser itself is not capable of modulating light it is necessary to use an acoustic optical modulator, for example. Therefore, laser cutting devices become very large and unwieldy. Moreover, if the wavelength is to be made shorter, currently the only choice available is a gas laser. For example, if an ultraviolet laser is used there is still a problem in that the life and stability of the laser are not satisfactory.
Methods for producing a substrate by injection molding require the pattern reproducibility to be in proportion to the density because a highly dense pattern is transferred, which causes technical difficulties. For example, the jitters of a reproduced signal depend on the accuracy of the cutting machine's original signal and the preciseness of the bit pattern on the substrate made by injection molding. Because of this, the allowable jitters have to be reduced to one quarter (1/4) as the recording density quadruples. However, there is a problem if no countermeasures are taken in that the signal-to-noise ratio (s/n ratio) of the reproduced signal decreases because changes in the pattern produced by the injection molding method are fixed, regardless of pattern size.
In the conventional art, in methods that use an original glass plate, the glass plate must have strict precision in flatness and planeness because the pattern formed on an optical disk is very fine and thus highly precise laser cutting is necessary. For that reason, the glass plate must have a thickness of at least a few mm and a diameter that is 1.5 times larger than the specified diameter of the optical disk to be produced, in order not to effect the uniformity of the plane. For example, when CDs (compact discs) with a diameter of 120 mm are produced, a glass plate with a diameter of the 200 mm level is used. Because of this, the original glass plate is heavier and in order to have high precision it is necessary to increase the size both of the rotating table of the spin-coater for coating the plate with a photoresist and of the laser cutting machine. In a production line, it is also necessary to have larger conveyor equipment to carry the glass plate. Furthermore, since the cost of a highly precise glass plate by itself is high, it is not disposed of and in practice it is always reused after regrinding the surface. Consequently, a regrinding process is also required. Accordingly, this method has the drawback that large scale production facilities are required and it is difficult to simplify the production process which results in high production costs.
In methods for producing a stamper by electroforming, it is necessary for the stamper to have a thickness of 0.2 mm or greater in order to obtain the strength required for a stamper. Therefore, this method has the drawbacks that production is time consuming and thus mass production is not applicable which results in high production costs.
Japanese laid-open patent publication No. 4-259937 recites a method that uses a silicon wafer to directly produce a stamper. This method has the drawback that because a silicon wafer is used to directly carry out the injection molding of a polycarbonate, the hard and fragile silicon wafer can be broken by high transfer pressure applied during injection molding (e.g. 20 tons) or by shear force applied when removing the mold.
Also, Japanese laid-open patent publications No. 61-68746, No. 4-310624 and No. 4-311833 provide improvements with respect to drawbacks that occur in methods using a glass plate by replacing a silicon wafer for the glass plate. But, they still cannot solve the problems caused in the conventional electroforming process because a stamper is produced from a silicon wafer by electroforming. However, it is possible to improve the recording density by methods recited in Japanese laid-open patent publication Nos. 4-310624 and 4-311833.
Further, Japanese laid-open publication No. 5-62254 provides a method that permits mass production while using neither a glass plate nor electroforming. In this method, after a pattern has been transferred to a sheet material, it is necessary that the sheet material be punched and an aluminum reflection film, photoelectromagnetic recording film or phase-changing recording film be formed on the substrate by vacuum metalizing. However, plastic substrates have the drawback that the degree of vacuum of the vacuum device drops due to moisture adsorption, etc. In the above injection molding, a substrate which has been molded at a high temperature is immediately guided to a vacuum device in order to deal with the problem. In this method, however, it is necessary to introduce a new step to let gas out of the substrate prior to vacuum deposition.
Alternatively, in cases where a polycarbonate used in conventional injection molding is used as the substrate material, if variations in the thickness of the light-curing resin coating occur or if microscopic dust is present in the light-curing resin coating, portions that have an unusual reflective index can be observed due to variations in the optical anisotropy of the polycarbonate. This is because contraction during curing does not occur evenly. As a result, servo failure occurs during reproduction of an optical disk or errors occur while recording or reproducing information. Because an optical disk has a thin film such as a reflection film or a recording film that is formed in a process under vacuum, absorbed gas or moisture on the substrate surface has to be fully removed. However, a polycarbonate has a high moisture permeability, and thus it has the drawback of not being ready for vacuum deposition unless it is heated or stored in a vacuum for a long period.
In injection molding, which is the most common conventional method for producing an optical disk, either case of using a polycarbonate or using a cyclic poly olefin resin is heated until it melts and then is poured into a mold under a high pressure. In this method, the mold temperature is set to a lower temperature than the melting temperature of these resins, and the resin is poured into a mold as it cools down. Therefore, the resin has high viscosity and as the pattern to be transferred becomes finer the transfer pressure and temperature have to be increased. However, if the resin temperature is raised too high, resin decomposition occurs which results in an increase of error when regenerating recorded signals, which is undesirable. In addition, the production speed drops significantly.